MUTATIONS IN THE LINKER REGION OF BVGS ABOLISH RESPONSE TO ENVIRONMENTAL SIGNALS FOR THE REGULATION OF THE VIRULENCE FACTORS IN BORDETELLA-PERTUSSIS

Expression of virulence factors of Bordetella pertussis is coordinatel y regulated by the products of the bvg locus, which codes for a sensor y protein (BvgS) and a positive regulator of transcription (BvgA), a p air in the family of bacterial 'two-component' regulators. Transcripti on of the bvg-regulated promoters is repressed by modulating environme ntal factors such as 50 mM MgSO4, 10 mM nicotinic acid (NA) or low tem perature (25 degrees C). We have isolated a spontaneous mutant (SK170) which expresses virulence genes at either 25 degrees C, or in the pre sence of 1-5 mM NA, or 10-50 mM MgSO4. Virulence factors in strain SK1 70 are still repressed by higher concentrations of NA (10 mM), or by a combination of low temperature (25 degrees C) and one of the other mo dulating agents. From this strain, we have isolated a second mutant (S K180) that showed constitutive synthesis of the virulence factors unde r any growth regime. Nucleotide (nt) and deduced amino acid (aa) seque nce analysis showed that SK170 contains a substitution at aa(570) of B vgS and SK180 contains an additional substitution at aa(680). These su bstitutions are confined to a 161-aa sequence that links the transmemb rane (TM) and kinase domains of BvgS. These mutations also alter the t ranscriptional autoregulation of the P-1 and P-2 promoters of the bvg locus. P-1, which in the wild-type (wt) strain is repressed by modulat ing agents, is constitutively active in the mutant strains. On the con trary, P-2, which is normally induced by all three modulating agents, is active in strain SK170 only in the presence of MgSO4 or NA, while i n strain Sk180 this promoter is repressed by modulating agents. The mu tants exhibit elevated levels of the BvgA regulatory protein and have a virulent phenotype also in the presence of modulating agents.